Traveling wave modulator

ABSTRACT

In an embodiment, an optical modulator comprising an optical path having at least one optical waveguide, and an impedance formed along the optical path, wherein the impedance comprises a capacitance that increases along the optical path. In another embodiment, a method for increasing bandwidth of an optical modulator by applying a first voltage applied to a beginning of a resistive line. and applying a second voltage applied to an end of the resistive line; wherein the first voltage is less than the second voltage.

RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Patent Application Ser. No. 62/467,788, filed Mar. 6, 2017 andentitled “TRAVELING-WAVE OPTICAL MODULATORS WITH VARYING IMPEDANCE,”which is hereby incorporated herein by reference in its entirety.

BACKGROUND

Optical transmission of information over a fiber optic cable encodes theinformation on a light wave.

BRIEF DESCRIPTION OF THE FIGURES

Various aspects and embodiments of the application will be describedwith reference to the following example embodiments. It should beappreciated that the figures are not necessarily drawn to scale.

FIG. 1 is a simplified illustration of a model of a modulator, inaccordance with an embodiment of the present disclosure;

FIG. 2 is an illustration of a model of a portion of a modulatorexpressed as a circuit diagram, in accordance with an embodiment of thepresent disclosure;

FIG. 3 is an alternative illustration of a model of a portion of amodulator expressed as a circuit diagram with increasing capacitance, inaccordance with an embodiment of the present disclosure;

FIG. 4 is a further alternative illustration of a model of a portion ofa modulator expressed as a circuit diagram with increasing capacitanceand increasing resistance, in accordance with an embodiment of thepresent disclosure;

FIG. 5 is a simplified illustration of a model of a modulator and across section of a p-n junction with waveguides implementing a portionof the modulator, in accordance with an embodiment of the presentdisclosure;

FIG. 6a is a graph illustrating electrical-to-optical response plottedfor a modulator with increasing capacitance, in accordance withembodiments of the present disclosure;

FIG. 6b is a graph illustrating electrical-to-optical response plottedfor a modulator with increasing capacitance and resistance, inaccordance with embodiments of the present disclosure;

FIGS. 7a and 7b are graphs showing inductance and capacitor seriesconductance vs. distance along a modulator, in accordance withembodiments of the present disclosure.

SUMMARY

In an embodiment, an optical modulator comprising an optical path havingat least one optical waveguide, and an impedance formed along theoptical path, wherein the impedance comprises a capacitance thatincreases along the optical path. In another embodiment, a method forincreasing bandwidth of an optical modulator by applying a first voltageapplied to a beginning of a resistive line. and applying a secondvoltage applied to an end of the resistive line; wherein the firstvoltage is less than the second voltage.

DETAILED DESCRIPTION

Generally, an optical wave modulator encodes information by applying awaveform to modulate a light wave. Typically, a wave modulator mayinclude a mach-zehnder interferometer, which may be a two armedinterferometer. Conventionally, an optical wave modulator may splitlight into two waveguides, modulate the light in each waveguide, thenrecombine the light into a single waveguide. Usually, a light wave maybe modulated by using an electrical field.

In some embodiments, a waveguide of a modulator may be silicon, whichtransitions to p and n doped regions. In most embodiments, applying afield across a p-n junction or p and n doped regions of a waveguide mayenable modulation of light. In many embodiments, applying a field acrossa p-n junction may change a depletion width of a waveguide through areverse bias. In most embodiments, applying more voltage may make adepletion width wider, which may result in a change in the amount ofelectrons and holes in the waveguide. In many embodiments, changing theamount of electrons and holes in a waveguide may change a refractiveindex of the waveguide.

In most embodiments, each waveguide of a modulator may be modulated in apush-pull fashion. In many embodiments, push pull may describe apositive voltage change in a first waveguide and a negative voltagechange in a second waveguide. In some embodiments, push pull maydescribe an increase of one depletion width and a decrease of anotherdepletion width.

In most embodiments, as a light wave travels down a modulator anelectric field may launch from a left side of the modulator and maytravel down the transmission line. In most embodiments, an electricalwave and optical wave of a modulator may have the same speed. In mostembodiments, at the end of an electrical line there may be a resistor toterminate an electrical field generated by the electrical line. In someembodiments, if an electrical field is not terminated, it may reflectback and cause problems.

Conventionally, today's modulators are limited in their bandwidth or theability to modulate the light. Typically, the faster the modulation ispushed, the more the electric field is dissipated. Generally, highfrequencies may not survive down a modulator very far. Usually, lowfrequencies may survive at an end of a modulator but high frequenciesare of significant value only at a beginning of a modulator.

In certain embodiments, it may be helpful to understand how a modulatorfunctions using a model or level of abstraction. In many embodiments, itmay be helpful to design a modulator by using a model of the modulator.In many embodiments, part of a modulator may be modeled as twowaveguides running through a set of diodes.

In many embodiments, a waveguide and ability to encode data may bemodeled using diodes, which may represent part of a modulator. In someembodiments, the number of segments of diodes used in the model may belarge in order to accurately model a modulator, where a modulator mayactually represent a contiguous portion of doped p-n junction. Infurther embodiments, each diode of a set of diodes may be modeled as anelectric circuit. In many embodiments, each electric circuit may containone or more inductors, resistors, and capacitors. In some embodiments,using a set of inductors, resistors, and capacitors to model amodulator, respective values of each of the inductors, resistors, andcapacitors may be the same.

In some embodiments, a model of a modulator may include an arrangementof inductors of inductance L and resistors of resistance RL interleavedwith an arrangement of capacitors of capacitance C and resistors ofresistance RC. In certain embodiments, an arrangement of some inductorsand resistors may be in series. In many embodiments, some resistors andcapacitors may be in a parallel arrangement. In some embodiments,arrangements of inductors and resistors and capacitors and resistors maybe repeated multiple times to model a modulator. In most embodiments, amodulator may terminate with a resistor of a given capacitance. In manyembodiments, multiple sets of inductors, capacitors, and resistors,arranged as a set of circuits may be used to model the behavior of amodulator, where the modulator may be continuous.

The current disclosure realizes that, in many embodiments, the bandwidthof a modulator may be limited as an electrical signal used to producethe modulation may become attenuated as it travels down the modulator.The current disclosure, in some embodiments, realizes that bandwidthlimitations of a traveling-wave silicon photonic modulator may be resultof dissipation of current in the capacitor feed resistance, R_(C), asthe RF wave feeds capacitors. In many embodiments, the currentdisclosure realizes that higher frequencies charge and discharge thecapacitors more often and thus may suffer higher attenuation as thecurrent passes through the resistors in and out of the capacitors. Inmany embodiments, a higher R_(C) and/or a higher C may result in higherattenuation. In certain embodiments, the current disclosure realizesthat there may be tradeoffs between having a lower the R_(C), the higherthe doping in the silicon and thus the higher the optical loss; and thelower the C often the weaker the modulation efficiency.

In other embodiments, the current disclosure realizes that it may bepossible to improve the performance of a modulator by varying values ofone or more inductors, resistors, and capacitors that may make up amodel of the modulator. In many embodiments, a model that increasesbandwidth may be implemented in a physical modulator. In certainembodiments, varying an impedance may offset attenuations, which mayincrease bandwidth of a modulator. In other embodiments, varying acapacitance may increase bandwidth in a modulator. In some embodiments,an optical modulator may have performance of 25-30 GHz.

In further embodiments, varying resistance along a waveguide mayincrease the bandwidth of a modulator. In some embodiments, increasingcapacitance running along a waveguide may increase the bandwidth of amodulator. In many embodiments, increasing resistance along a waveguideof a modulator may increase the bandwidth of the modulator. In certainembodiments, increasing resistance and capacitance along a waveguide ofa modulator may increase the bandwidth of the modulator. In someembodiments, bandwidth of a modulator may be increased by loweringelectrical loss for high frequencies at the beginning of the modulator.In certain embodiments, efficiency at a beginning of a modulator may betraded for increased bandwidth.

In some embodiments, by having R_(C) and C start small and then increasealong the modulator, lower high-frequency loss at the beginning of themodulator and high high-frequency loss at the end of the modulator maybe obtained. In certain embodiments, a same total high-frequency lossmay be experienced while larger net modulation at frequencies may beprovided. In many embodiments, higher modulation bandwidth may beobtained without sacrificing the total optical loss and the totalmodulation efficiency.

In many embodiments, a change in impedance may be achieved by varyingthe capacitance of a traveling-wave optical modulator. In certainembodiments, a change in capacitance may be achieved by varying thedoping concentration of a phase shifter (e.g., a p-n-junction). In otherembodiments, a change in capacitance may be achieved by varying a sizeof a phase shifter (e.g., a p-n-junction). In further embodiments, achange in capacitance may be achieved by varying bias voltage along themodulator. In some embodiments, a change in impedance may be achieved byvarying the resistance of a traveling-wave optical modulator. In certainembodiments, a change in resistance may be achieved by varying thedoping concentration in a waveguide. In some embodiments, a change inresistance may be achieved by moving a doped region closer to or awayfrom the optical wave guided by the optical waveguide.

In many embodiments, moving the doping further from the optical wave mayincrease the resistance. In some embodiments, an optical modulator maybe created by locating a doped region of the waveguide closer to theguided optical wave by 0.1 to 1.0 microns. In certain embodiments, inaddition to a pn doping there may be a P+ and N+ doping in a waveguide.In certain embodiments, P+ and N+ doping may be at the edge of awaveguide. In some embodiments, a waveguide may be 0.5 microns in width.In some embodiments, P++ and or N++ doping may be at 0.4 microns. Incertain embodiments, P++ and/or N++ doping may be varied from 0.1 to 0.8microns. In some embodiments, metal lines, such as conductive metallines may be 20 microns wide. In certain embodiments, metal lines, suchas conductive metal lines may be separated by a 15 micron gap. In manyembodiments, the width of a metal line may be varied to between 10-15microns. In some embodiments, the gap of metal lines may be varied tobetween 15-30 microns.

In some embodiments, there may be multiple ways to increase C and R_(C)along a modulator. In a particular embodiments, capacitance C may beincreased by increasing doping in the center of a waveguide. In someembodiments, the higher the doping the smaller the depletion region maybe, the higher the capacitance may be. In certain embodiments,capacitance C may be increased by changing the modulator bias along themodulator. In some embodiments, a bias voltage may be higher at thebeginning of the modulator than at the end of a modulator to increasebandwidth. In some embodiments, R_(C) may be increased by decreasingand/or moving away the doping in or near the waveguide to increasebandwidth. In certain embodiments, to decrease the doping, multiplesegments may be used to increase bandwidth.

In some embodiments, a modulator's inductance may be varied along themodulator's length. In many embodiments, inductance may be larger at thebeginning of a modulator and may be decreased toward the end of themodulator. In certain embodiments, inductance may be increased byincreasing a gap between metal lines and/or by narrowing a width of themetal lines. In most embodiments, variations in inductance may result invariations in impedance.

In many embodiment, bandwidth of a modulator may be increased by using amodulator that has less electrical loss at the beginning of themodulator than occurs at the end of the modulator for high frequencies.In some embodiments, a different bias voltage may be used at thebeginning and the end of a modulator. In most embodiments, a higher biasvoltage may be used at the beginning of a modulator than at the end of amodulator. In certain embodiments, there may be a tradeoff betweenhigher optical power loss and higher modulation or more bandwidth. Insome embodiments, varying impedance may be used to increase bandwidth ina silicon photonic modulator. In other embodiments, varying impedancemay be used in other types of optical modulators. In furtherembodiments, varying impedance may be used in an indium phosphideoptical modulator. In some embodiments, techniques of the currentdisclosure may be with other traveling-wave modulator designs, such ascapacitive loaded traveling-wave modulators.

Refer now to the example embodiment of FIG. 1, which illustrates amodulator in accordance with an embodiment of the current disclosure. Inthis embodiment, splitter 101 splits the light into waveguide 102 and104. Waveguide 102 passes through diodes 110 and waveguide 104 passesthrough diodes 120. Waveguides 102 and 104 recombine at coupler 135.Resistive line 127 passes between and connects diodes 110 and diodes 120and serves as an input for bias voltage V1 125 and voltage V2 130. Line117 connects to diodes 110 and line 119 connects to diodes 120. Lines117 and 119 are connected with resistor 140, which is used to terminatean electric field in the modulator. Modulator 100 modulates light byapplying bias voltages V1 125 and V2 130 through resistive line 127. Aslight travels through waveguides 102 and 104, the bias voltages cause anelectrical field to occur in Modulator 100 modulating the light inwaveguides 102 and 104.

Refer now as well to the example embodiment of FIG. 2, which representsan example model of the diodes 110 of modulator 100. In this exampleembodiment, circuit 205 corresponds to diodes 105 and 107 of Modulator100 and circuit 110 corresponds to diode 115 of Modulator 100. Theellipse between circuit 205 and 210 represents the other diodes ofdiodes 110 of modulator 100; although not shown in FIG. 2 wouldrepresent the same or similar circuits. Circuit 205 has inductor 215 inseries with resistor R1 220 which is in series with inductor 225 andresistor 230. Capacitor 240 and resistor R_(C) 245 are in parallel withinductor 215 and with resistor 235 and inductor 250. Circuit 210 hasinductor 255 and resistor 260 in parallel with resistor 265 andcapacitor 270. Terminal resistor RT 275 corresponds to resistor 140 inmodulator 100 and is used to terminate an electrical field. In theembodiment of FIG. 2, each of inductors 215, 225, and 255 have the samevalue. Each of resistors 220, 235, 245, 260 and 265 have the samevalues. As well, capacitors 240, 250, and 270 also have the same value.A modulator represented by circuits 205 and 210 has a constant impedancealong it length. As well, turning back to FIG. 1, diodes 120 may also berepresented by a set of circuits such as those shown in FIG. 2.

Refer now to the example embodiments of FIGS. 1 and 3, where FIG. 3represents an alternative model of the diodes 110 of modulator 100. Inthis example embodiment, circuit 305 corresponds to diodes 105 and 107of Modulator 100 and circuit 310 corresponds to diode 115 of Modulator100. The ellipse between circuit 305 and 310 represents the other diodesof diodes 110 of modulator 100. In this embodiment, for additionalcircuits between Circuit 305 and 310 the capacitor in parallel for eachadditional circuit would have an increasing capacitance between thecapacitance of capacitor 350 and capacitor 370. Circuit 305 has inductor315 in series with resistor R1320 which is in series with inductor 325and resistor 330. Capacitor 340 and resistor R_(C) 345 are in parallelwith inductor 315 and with resistor 335 and inductor 350. Circuit 310has inductor 355 and resistor 360 in parallel with resistor 365 andcapacitor 370. Terminal resistor RT 375 corresponds to resistor 140 inmodulator 100.

In the embodiment of FIG. 3, each of inductors 315, 325, and 355 havethe same value. Each of resistors 320, 335, 345, 360 and 365 have thesame values. In the example embodiment of FIG. 3, capacitor 340 has asmaller value than capacitor 350, which in turn has a smaller value thancapacitor 370. A modulator represented by circuits 305 and 310 has anincreasing capacitance along it length. In many embodiments, anincreasing capacitance such as in FIG. 3 may lead to a higher bandwidthan may have a weaker modulation efficiency. As well, turning back toFIG. 1, diodes 120 may also be represented by a set of circuits such asthose shown in FIG. 3.

Refer now to the example embodiments of FIGS. 1 and 4, where FIG. 4represents an alternative model of the diodes 110 of modulator 100. Inthis example embodiment, circuit 405 corresponds to diodes 105 and 107of Modulator 100 and circuit 410 corresponds to diode 115 of Modulator100. The ellipse between circuit 405 and 410 represents the other diodesof diodes 110 of modulator 100. In this embodiment, for additionalcircuits between Circuit 405 and 410 the capacitor in parallel for eachadditional circuit would have an increasing capacitance between thecapacitance of capacitor 450 and capacitor 470. In this embodiment, foradditional circuits between Circuit 405 and 410 the resistor in parallelfor each additional circuit would have an increasing resistance betweenthe resistance of resistor 445 and resistor 465. Circuit 405 hasinductor 415 in series with resistor R1 420 which is in series withinductor 425 and resistor 430. Capacitor 440 and resistor R_(C) 445 arein parallel with inductor 415 and with resistor 435 and inductor 450.Circuit 410 has inductor 455 and resistor 460 in parallel with resistor465 and capacitor 470. Terminal resistor RT 475 corresponds to resistor140 in modulator 100. In the embodiment of FIG. 4, each of inductors415, 425, and 455 have the same value.

In the example embodiment of FIG. 4 resistors 440 has a smallerresistance value than resistor 450, which has a smaller resistance valuethan resistor 465. In the example embodiment of FIG. 4, capacitor 440has a smaller value than capacitor 450, which in turn has a smallervalue than capacitor 470. A modulator represented by circuits 405 and410 has an increasing capacitance along it length. A modulatorrepresented by circuits 405 and 410 has an increasing resistance alongits length. In many embodiments, an increasing capacitance such as inFIG. 4 may lead to a higher bandwidth an may have a weaker modulationefficiency. As well, turning back to FIG. 1, diodes 120 may also berepresented by a set of circuits such as those shown in FIG. 4.

Refer now to the example embodiment of FIG. 5, which illustrates amodulator with a waveguides passing through diodes and a correspondingrepresentation of the diodes and waveguide as p-n junctions. Portion 505of modulator 500 is represented as cross section 510. Cross section 510illustrates how a modulator may be implemented in silicon. Diodes 517and 519, as well as resistive line 527 and conductive lines 507 and 509.Waveguides 502 and 504 in modulator 500 are represented as waveguides502 and 504 in cross section 510. Metal contacts 515, 520, and 525 arepresent both in portion 505 of modulator 500 and cross section 510. Inmost embodiments, to implement the full length of modulator 500, crosssection 510 would be stretched to the length of the modulator and metalcontacts reproduced on the other side of cross section 510, generally asshown in modulator 500.

In many embodiments, give a modulator such as that of cross section 510,there may be different ways to increase bandwidth of the modulator. In aparticular embodiment, capacitance C may be increased by increasing thedoping in the center of the waveguide and/or changing the modulator biasalong the modulator. In other embodiments, a bias voltage may be higherat the beginning of the modulator such as the metal contacts 504 ofmodulator than at the end of the metal contacts such as that of 525 ofmodulator 500. In still other embodiments, multiple sections, such asthat of cross section 510 may be used to increase doping of a modulator.In certain embodiment, inductance may be increased by increasing the gapbetween the metal lines such as lines 507 and 509 of modulator 500. Inother embodiments, inductance may be increased by increasing the gapbetween the metal lines such as lines 507 and 509 of modulator 500 bynarrowing the width of the metal lines.

Refer now to the example embodiment of FIGS. 6a , which illustrateelectrical-to-optical response plotted for a modulator with increasingcapacitance. Refer now to the example embodiment of 6 a, whichillustrates electrical-to-optical response plotted for a modulator withincreasing capacitance and resistance.

Refer now to the example embodiments of FIGS. 7a and 7b , whichillustrate capacitance and conductance (the inverse of resistance RC)along a modulator resulting in an increase of bandwidth. In theseexample embodiments which illustrate an implementation of FIG. 4, C=2.6pF/cm, GL=8×10−4 mho/m. The optical group index is 3.9. The terminationresistance is 50Ω. The modulator length is 4 mm, and the modulatorcomprises 100 segments. In these example, the source is a current sourcewith 350-Ω and a 70-fF capacitor in parallel. In this example, thevariation of capacitance C provides a 6-GHz bandwidth improvement, andthe variation of resistance RC improves the bandwidth even further athigher frequencies.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications, and improvements areintended to be within the spirit and scope of the technology describedin the application. It is, therefore, to be understood that theforegoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed. In addition, any combination of two or more features,systems, articles, materials, and/or methods described herein, if suchfeatures, systems, articles, materials, and/or methods are not mutuallyinconsistent, is included within the scope of the present disclosure.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified.

What is claimed is:
 1. An optical modulator comprising: an optical pathhaving at least one optical waveguide; and an impedance formed along theoptical path, wherein the impedance comprises a capacitance per unitlength that increases along the optical path.
 2. The optical modulatorof claim 1, wherein the capacitance is formed by part of one or morepn-junctions.
 3. The optical modulator of claim 2, wherein the impedancecomprises a resistance that increases along the optical path.
 4. Theoptical modulator of claim 3 further comprising a resistive elementformed along the optical path.
 5. The optical modulator of claim 4further comprising a resistive line; wherein the resistive line has afirst voltage applied to a beginning of the resistive line and whereinthe resistive line has a second voltage applied to an end of theresistive line; wherein the first voltage is less than the secondvoltage.
 6. The optical modulator of claim 5 wherein the change inresistance is created by varying a doping concentration in a waveguide.7. The optical modulator of claim 4 wherein the resistance is formed bypart of one or more pn-junctions.
 8. The optical modulator of claim 3wherein the change in resistance is created by increasing doping in thewaveguide.
 9. The optical modulator of claim 8 wherein ends of theresistive lines are metal lines and are connected by a resistor.
 10. Theoptical modulator of claim 3 wherein the change in resistance is createdby locating a doped region of the waveguide closer to the guided opticalwave by 0.1 to 1.0 microns.